1. Field of the Invention
The present invention relates to a small-sized semiconductor wavelength conversion device having a wide operating wavelength range, in which a bistable laser is used.
2.Description of the Prior Art
An embodiment of conventionally known bistable type semiconductor wavelength conversion device is shown in FIG. 8 as a structural, cross-sectional view. In this figure, reference numeral 1 represents an n-type substrate of, for instance, n-InP; 2 an n-type semiconductor cladding layer of, for instance, n-InP formed on substrate 1; and 3, 4 and 5 a saturable absorber region, a gain region and a distributed Bragg reflector (DBR) mirror having a corrugation grating for selecting a wavelength of wavelength-converted light, which are formed on cladding layer 2. These waveguide layers 3 to 5 can be formed from InGaAsP. A p-type semiconductor cladding layer 6 of, for instance, p-InP is disposed on these regions 3 and 4 and corrugation grating 5 to give a ridge waveguide. An electrode 7 for the saturable absorber region, an electrode 8 for the gain region and an electrode 9 for the distributed Bragg reflector are arranged in desired areas on cladding layer 6 which correspond to the areas of regions 3 and 4 and corrugation grating 5, respectively. An electrode 10 is arranged on the opposite main face of substrate 1. Thus, the device is designed such that input light passing through an optical isolator 100 having a wavelength of .lambda..sub.1 is made incident upon region 3.
In this device, an electric current is injected into active region 4 through electrode 8 for the gain region so that active region 4 has an optical gain and the wavelength conversion output light .lambda..sub.x is performed by input light .lambda..sub.1. Simultaneously, wavelength .lambda..sub.x of the wavelength conversion output light is controlled through the change of the refractive index of distributed Bragg reflector mirror portion 5 by the injection of an electric current through tuning electrode 9.
The behavior of this device observed when a signal light having a wavelength .lambda..sub.1 is made incident upon the device can be explained as follows (see S. Yamakoshi et al., Postdeadline Papers of OFC'88, PD-10 (1988)).
An output light of the bistable type wavelength conversion device having saturable absorber region 3 has an ON/OFF characteristic in which an output light is turned ON/OFF in response to a strength of an incident signal light when the loss in absorber region 3 is modulated by the incident signal light. The wavelength of the output light is determined by the Bragg wavelength of corrugation grating 5. Therefore, an electric current injected into electrode 9 can change the refractive index of corrugation grating 5, so that the output light wavelength can be changed. Accordingly, the output signal light controlled to have a wavelength different from that of the input signal light can be obtained. This is just the operation principle of the wavelength conversion device shown in FIG. 8.
The practical operation of this device requires the use of input power of not less than about 15 .mu.W for an InGaAsP device operating at about 1.5 .mu.m band and thus devices whose wavelength conversion range is not less than about 4.5 nm can be obtained.
Since such a conventional wavelength conversion device can emit conversion light in both forward and backward directions, the optical isolator 100 must be used in the conventional wavelength conversion device in order to prevent the influence of returned conversion light on preceding stage. The optical isolator is composed of a non-reciprocal substance and, therefore, cannot be incorporated into optical IC's. This causes fatal drawbacks when the wavelength conversion devices are integrated on the same wafer, since it is difficult to incorporate the optical isolator into a semiconductor wafer.
As has been discussed above, if a communication system is completely constituted from optical devices, the use of a large scale frequency switch is required, but such a large scale frequency switch cannot be obtained by simply combining conventional independent devices and, therefore, there has long been desired for the development of such a switch in the form of an optical IC.